Surface preparation of uranium parts

ABSTRACT

Prior to electroplating parts of uranium or uranium base alloys, the parts are subjected to a surface preparation which comprises preliminary steps of chemical degreasing, electrolytic degreasing and if necessary of chemical etching and further comprises an anodic electrolytic treatment in an aqueous solution of an organic salt or a sulphate of lithium and/or magnesium.

9 firmed States Patent [15] 3,674,655

dc Vaulchier du Deschaux et al. 1 July 4, 1972 [54] SURFACE PREPARATION OF [56] References Cited URANIUM PARTS UNITED STATES PATENTS [72] Inventors: Louis de Vaulchier du Deschaux, Paris;

l i de Fouchier Bourg La Reine; 2,853,441 9/1958 Flll'lt ..204/l.5 manue] de vaucelles, p Louis Facquet, 2,872,387 2/1959 Kolodney vaucresson all of France Walker et al. [73] Assignee: gommssariat A LEnergie Atomique, Primary Examiner Reuben Epstein fame Att0rney-Cameron, Kerkam and Sutton [22] Filed: Jan. 13, 1969 211 App]. No.: 790,869 [57] ABSTRACT Prior to electroplating parts of uranium or uranium base al- [30] Foreign Application Priority Data loys, the parts are sub ected to a surface preparat on which comprises preliminary steps of chemical degreasmg, elec- Jan. 24, 1968 France ..68l373l2 trolytic degreasing and if necessary of chemical etching and further comprises an anodic electrolytic treatment in an aque- [52] U.S. Cl ..204/1.5, 204/141 ous l ti f a r anic alt or a sulphate of lithium and/or [51] Int. magnesium [58] Field of Search ..204/1.5

8 Claims, No Drawings SURFACE PREPARATION OF URANIUM PARTS The present invention relates to surface preparation of parts formed of unalloyed uranium or of uranium rich alloys such as those which contain Va, Zr, Pu, Mo, Cr, etc such surface preparation is necessary prior to forming metallic coating and in particular electrolytic coatings on the parts.

It is old in the art to deposit electrolytic coatings on articles or parts whose superficial part at least is electrically conductive. The parts are placed in baths having an appropriate composition and are connected to the negative pole of a direct current source. The positive pole of the source is in turn connected to an electrically conductive anode immersed in the bath.

In order to ensure that electroplating work is correctly performed and results in a satisfactory coating (that is to say an adherent and continuous coating having a close dimensional tolerance), surface preparation of the base metal to be coated is essential. The base metal may of course consist of a material constituting the whole of the part or of a conductive coating previously deposited on the part.

An important purpose of the surface preparation is to clean or scrape the part to be plated and to remove any layer of foreign substances such as grease, oxides, scale and the like which may normally be present on the surface of the base metal. in the case of ordinary metals such as iron, copper or their alloys, a number of different conventional methods of preparation produce satisfactory results. One method which is commonly employed starts with chemical degreasing by means of an organic solvent such as trichloroethylene, electrolytic degreasing carried out at the cathode of a sodium hydroxide bath, washing and pickling. In the case of ferrous metals, an additional process which can be mentioned by way of example consists of anodic pickling in an aqueous sulphuric acid solution. In the case of copper-containing metals, the process adopted consists of chemical pickling in a solution containing nitric acid as well as a number of different constituents which may be required. It has also been proposed to carry out electrolytic pickling in strongly acid aqueous solutions of various salts, especially salts of alkali metals. After the pickling process in which the surface of the metal to be treated is completely exposed, the part is washed and the plating operation is then carried out; in the case of electrodeposition of one metal, the part which is withdrawn from the washing bath is immediately placed in the plating bath which corresponds to the desired metal.

The application of the theoretical processes described in the foregoing is no longer as simple as in conventional practice when the material to be plated is a metal which oxidizes or corrodes very readily. in such a case, layers of products resulting from the pickling process would remain at the surface after the final washing as well as ions which would subsequently give rise to chemical attack at a correspondingly faster rate as the concentration of the pickling bath is higher. This is the case in particular of alloyed or unalloyed uranium.

A number of different expedients have already been proposed for circumventing the disadvantages attached to the methods employed in the prior art. Recourse has been had in particular to chemical processes, one of which is described in French Pat. No. 1,523,216 filed by Commissariat a lEnergie Atomique and produces satisfactory results.

The process referred to comprises the following operations:

degreasing by means of trichloroethylene;

electrolytic cathodic degreasing for a period of 3 minutes at a current density of 5 to A/dm in an alkaline solution containing 40 g/l of sodium hydroxide;

washing;

pickling with 10 N nitric acid at ambient temperature;

further washing;

pickling with 10 N hydrochloric acid at ambient temperature, whereupon the part is withdrawn from the hydrochloric acid in order to allow attack of the metal to develop in air;

further washing;

further pickling with 10 N nitric acid at ambient temperature;

final washing.

This chemical cleaning and pickling process permits the formation of highly adherent electrodeposited coatings on the cleaned surface of the uranium but is nevertheless subject to a number of drawbacks. in particular, the base-metal surface which is obtained as a result of this cleaning process does not have a metallic appearance but remains covered with a greyish film which can be considered to have a detrimental eflect on the quality of adhesion of the plated coating on the uranium although a good standard of adhesion is nevertheless achieved. Moreover, the film referred to can in some instances retain water or traces of baths in which the part has been treated and therefore can subsequently cause attack of the uranium beneath the plated coating, such attacking action being in turn liable to result in loss of adhesion or even in destruction of the coating. The attack of the pure or alloyed uranium by the different baths mentioned in the foregoing is variable; in particular, the sensitivity to different reagents is dependent on the alloy being treated and consequently makes it necessary to adapt the concentrations and periods of attack in the case of each alloy. The resultant chemical action can frequently be violent, is sometimes irregular, and can give rise to dissolution of the base metal at variable depths, to uncertainty in regard to compliance with dimensional requirements and in regard to the weight of remaining uranium of treated parts and therefore results in a dispersion of the results ob tained. in consequence, it becomes necessary to carry out a minute inspection of the treated parts.

In addition, the surface of the base metal is rough and irregular after chemical attack, especially in the case of alloys. This is detrimental to the quality of the electrolytic coatings which are deposited and especially to their imperviousness, thus reducing the value of such coatings from the point of view of protection against corrosion.

Some attempt has been made to overcome these disadvantages by developing a chemical method of pickling which makes use of the same basic processes but has been studied so as to be less violent. The nitric acid employed is reduced from a concentration of 10 N to a concentration of 7 N and the hydrochloric acid is replaced by a pickling bath which is employed at ambient temperature and contains 40 g/l of cupric chloride (CuCl 2H O) and 1 g/l of concentrated hydrochloric acid.

After the initial treatment with nitric acid, the part is dipped in the pickling bath for a period of two minutes, then washed, dipped in nitric acid for another period of 2 minutes, washed, dipped in the pickling bath for a further period of 1 minute, then washed followed by further dipping nitric acid for 2 minutes, then finally subjected to final washing and placed in the electroplating bath. This process is less aggressive and makes it possible to obtain a chemical attack which is less violent than the method of preparation described in the French Pat. No. 1,523,216 cited earlier.

The inventors have also given consideration in the past to a number of different methods which are based on electrolytic anodic attack and intended to reduce the above-mentioned disadvantages even further. In particular, they have developed a method based on the use of sodium acetate which comprises the following steps:

degreasing by means of trichloroethylene or by means of perchloroethylene; electrolytic cathodic degreasing for a period of 3 minutes at a current density of 10 A/dm in a sodium hydroxide solution having a concentration of 40 g/l;

washing;

chemical pickling in 7 N nitric acid for a period of 1 minute at 25 C.;

further washing;

electrolytic anodic pickling in a sodium acetate solution containing 400 g/l of NaCH CO 3H O having a pH of 6 (adjustment of the pH value being carried out by means of acetic acid) at a current density of 7 A/dm and for a period of 15 minutes at 25 C.

further washing;

pickling with 7N nitric acid at 25 C for a period of l minute;

final washing.

The part which is delivered from the washing stage is then placed in the electroplating bath (nickel-plating bath, for example) and the plating operation is immediately begun.

The electrolytic pickling process last mentioned constitutes a substantial improvement over comparable processes of the prior art. In this process, the cleaned part has a much lower degree of surface roughness, the appearance of the cleaned surface is more uniform, the quantity of metal removed in the pickling process is much more readily controlled by adjusting the current density and the duration of the process and the dimensional changes which result from the pickling process can be predetermined with a better approximation.

However, this solution does not completely eliminate the defects of processes employed in the prior art: the surface of the base metal which results from the final washing still has some surface irregularities which may be expected to impair the strength of the plating. In order to ensure that the surface is stripped of all the chemical attacking products (especially nitric acid), a relatively high current density must be adopted. Failure to take this precaution would be liable to result in pitting due to chemical attack. Accordingly, the object of the invention is to remove or at least to attenuate these defects even further to yet a very appreciable extent.

To this end, the 'mvention proposes a method of surface preparation which comprises at the same time as conventional steps of degreasing, rinsing and possibly of chemical pickling anodic pickling in an aqueous solution containing an organic salt or a sulphate of lithium and/or magnesium.

This method provides a marked improvement over the methods described in the foregoing by virtue of the fact that, instead of only employing anions for the electrolytic anodic pickling process as has been the practice up to the present time, the invention utilizes at the same time the cation of the salt employed. The results are radically different from those obtained as a result of the use of other cations such as the acetates of sodium, potassium, ammonium, zinc, cadmium and so forth. This difference probably arises from the particular properties of the lithium and magnesium cations and from the fact that the salts which are fonned at the moment of pickling pass readily into solution in contradistinction to the salts which are formed with the other cations and which continue to adhere to the surfaces of the treated parts or which are in suspension in the solution.

It has been noted in particular that, among the various solutions of lithium and magnesium salts, the acetates produce a highly satisfactory cleaning action.

In the majority of cases, it can considered that satisfactory results are obtained in alloyed or unalloyed uranium by carrying out the anodic pickling process in a 2 N to 3 N solution of lithium acetate having a pH of 6 at 3 to 7 A/dm" and between 10 C. and 50 C. but preferably at about 25 C. for a period of time which is variable according to the current density, namely of the order of 35 minutes at 3 A/dm and of the order of minutes at 7 A/dm in practice, this current density must remain between 2 and A/dm".

The pickling process under consideration can be preceded by the following operations:

chemical degreasing by means of trichloroethylene or by means of perchloroethylene;

electrolytic cathodic degreasing for a period of 3 minutes at 10 A/dm in a solution containing 40 g/l of sodium hydroxide in the cold state;

washing;

chemical pickling in 7 N nitric acid for a period of 1 minute at C.; washing.

The two operations last mentioned are only essential if the part is highly oxidized in order that the anodic pickling process should have to remove only a relatively thin film. If the surface of the part to be treated is only slightly oxidized, these operations can be dispensed with.

Anodic pickling can be followed by:

chemical pickling in 7 N nitric acid at 25 C. from a few seconds to 1 minute;

a washing operation.

Finally, the part which is withdrawn from the final washing bath must immediately be placed in the plating bath through which a current is passed.

This method appears to be in general in scope and to be applicable to depleted, natural or enriched uranium as well as to the majority of uranium alloys.

It is found that, alter completion of the anodic pickling process, there only remains on the uranium a thin film (instead of the thick layer which remained after the treatments known heretofore). This film is more readily removed or reduced during subsequent operations and reveals the structure of the base metal. As is clear from the foregoing, the density (and correlatively the potential) of the currents employed in the anodic pickling process is distinctly lower than the values which were formerly considered necessary, with the result that the attack is less violent, that the thickness of the uranium layer which is removed as a result of pickling is small, uniform and readily controlled, that the final dimensions of parts can be determined prior to treatment and that the surface properties which a part may have acquired during previous metallurgical processes (especially work-hardening) are retained. By reason of the small amount of metal dissolved, the bath can be employed for a much greater number of operations and precipitates a much smaller quantity of insoluble substances, in particular when lithium salts are employed. This last aspect is significant when there is an attendant danger of criticality.

The nitric acid attacks which essentially preceded and followed anodic pickling in the process of the prior art can be attenuated and in some cases even dispensed with altogether.

The state of surface both of the base metal and of the electrolytic coating formed on the base metal are improved to a very substantial extent, the plated coating which is obtained has a much more uniform structure and, for equal thickness, the continuity and imperviousness of said coating are highly improved. From the point of view of protection against corrosion, this coating is of considerably greater value than the coatings obtained up to the present time. Adhesion of the plated coating on uranium is at least comparable with the coatings obtained by other methods but surface roughness has been reduced to a considerable extent and this in turn results in an improvement in the protective value of the coating.

The processing steps involved in the preparation of cylindrical parts formed of alloyed uranium prior to nickel-plating and tin-plating according to the method described in the abovecited French Pat. No, 1,523,216 will now be described by way of example.

Example 1 (Magnesium acetate bath):

The part was a cylinder of uranium alloy containing 1.5 percent molybdenum, having a diameter of 40 mm and a length of 299 mm, a surface area of 400 cm and a weight of 6,905 grams. A threaded bore having a diameter of 10 mm and a depth of 15 mm was formed at one end of the part and along the axis of the cylinder. A threaded rod which served to support the part and to supply current to this latter was screwed into said bore; the entire surface of the uranium alloy could thus be immersed. All sharp edges and comers of the part were rounded ofi so as to have a radius of 2 mm.

The part thus prepared was degreased by means of trichloroethylene for a period of 2 minutes, then placed at the cathode in an electrolytic degreasing bath consisting of a normal sodium hydroxide solution for a period of 3 minutes at 40 amps, namely A/dm the appearance of the surface remained unchanged and this operation resulted in an insignificant weight loss of the order of 10 mg.

After washing with water, the part was subjected to chemical pickling for a period of one minute in a 7 N nitric acid solution at 21 C. No apparent reaction was observed and the weight loss of the part was approximately 20 mg.

After washing, anodic pickling was performed with agitation in a solution of magnesium acetate having a 2 N concentration of M at a current density of 7 Aldm the operation being carried at 21 C. for a period of minutes; the initial pH of the solution was 6.00 and the final pH was 6.01.

During this operation, a slight evolution of gas from the part was observed. This was accompanied by the formation of a black film which dissolved in the solution and this latter turned yellow. The voltage and current intensity remained stable throughout the operation. On completion of the operation, the part was covered with a black film. This film was detached during the following operation which consisted in washing with running water and thus exposed the surface of the metal wlu'ch exhibited a grey color. During this pickling process, the part lost 10.43 g and the thickness of metal removed from the cylindrical surface was 14 microns.

The part was then pickled in a 7 N nitric acid solution at ambient temperature for a period of 1 minute. During this operation, a few remaining traces of the black film were dissolved and the weight loss was approximately 40 mg.

The part was then carefully washed with cold running water and placed as rapidly as possible in a nickel-plating bath through which a current was passed.

Example 2 (Lithium tartrate bath):

The same method of operation was employed for the treatment of a part made of the same alloy and having the same characteristics from the beginning of operations until the nitric acid pickling step which preceeds the anodic pickling.

The anodic pickling operation was then carried out for a period of 15 minutes in a lithium tartrate solution having a Li concentration of 2 N at a current density of 7 A/dm and at a temperature of 21 C. Agitation of the solution was maintained. The initial pH of the solution was 6.00 and the final pH was 6.01.

The same observations were made as in the case of Example 1. The black film was removed during the subsequent washing operation and more readily than in the case of pickling with magnesium acetate; the pickled subadjacent metal then exhibited a grey color. The weight lost by the part during this operation was 10.18 g.

On completion of the washing operation, the part was dipped for a period of 1 minute in a 7 N nitric solution, then carefully washed and placed in the plating bath as indicated in Example 1.

Example 3 (Lithium acetate bath):

A method of operation which is similar to the preceding was applied to a cylindrical part having a diameter of 35 mm, a length of 4 mm, a weight of 72.13 g and formed of uranium containing 0.2 weight percent of vanadium. The part which was degreased with trichloroethylene for a period of 2 minutes was placed at the cathode in a normal sodium hydroxide bath at 2.4 Amps (namely 10 A/dm) for a period of 3 minutes.

The appearance of the surface remained unchanged and the operation resulted in only a negligible weight loss (less than 1 milligram).

The part was washed then pickled for a period of 1 minute at 21.5" C in a 7 N nitric acid solution. No apparent reaction was observed and the weight loss was 4.8 mg.

Afier washing, anodic pickling was then carried out at 3 A/dm in a lithium acetate solution having a Li*' concenu'ation of 3 N and maintained with agitation for a period of 35 minutes; the temperature was 21 C., the initial pH was 6.00 and the final pH was 6.03.

During the first minutes of this treatment, a slight evolution of gas was noted. No visible film was fonned on the surface; the lithium acetate solution had changed to a yellowish tint. The part had lost 615 mg during this operation; the depth of chemical attack was 14 microns and the macrostructure of the alloy was revealed.

The part was washed, dipped in a 7 N nitric acid solution for a period of 1 minute (this operation resulted in the dissolution of 21.6 mg), was again carefully washed and finally placed in the plating bath as indicated in Example 1.

Example 4 (Lithium sulphate bath):

A similar mode of operation was applied to a uranium part containing 1.5 percent molybdenum, having a diameter of 35 mm, a length of 4 mm and a weight of 70. 16 g. Electric current was supplied through a threaded rod which was screwed into the part.

The part was degreased for a period of 2 minutes in toluene, subjected to a cathodic degreasing for 3 minutes in a normal sodium hydroxide solution at a current density of 10 A/dm, then washed and finally pickled for one minute in a 7 N nitric acid solution at 23 C.; no apparent reaction was observed and the weight loss was 1.8 mg.

Anodic pickling was carried out on completion of the washing operation. The part was subjected to vertical vibrations and immersed in a lithium sulphate solution having a Li concentration of 4 N, said solution being brought to a pH of 0 by addition of H 80, pickling was carried out at 6 A/dm at 23 C. for a period of 20 minutes.

At the beginning of this treatment, a fairly substantial evolution of gas was observed. The part was covered with a light black film which was almost entirely removed during the following washing operation. The solution had turned yellow and its final pH was 0.2. During this pickling operation, the part had lost 718 mg corresponding to a depth of 16 microns. After washing, the part was dipped for one minute in a 7 N nitric acid solution. This operation resulted in a weight loss of 3.8 mg. The part which was withdrawn from the nitric acid solution had a pale grey color, the macrostructure being revealed.

The part was carefully washed and immediately placed under tension in a nickel-plating bath.

Instead of either acetates (as in Examples 1 and 3) which nevertheless have the advantage of being readily available or tartrates (as in Example 2), many other salts of lithium and of magnesium are open to selection, in particular salts in which the anions can form soluble salts or complexes with uranium. Effective results are obtained by making use of organic acid salts such as the citrates, oxalates, lactates, fonniates, malonates, pyrogallates and succinates. The values of the different parameters vary only slightly with respect to those indicated earlier in regard to lithium acetate. The sulphates of magnesium and of lithium (Example 4) can also be employed. On the other hand, the use of lithium nitrate or magnesium nitrate would produce completely different results of the same kind as those obtained by nitric acid attack (which may form part of the chemical pickling which precedes the anodic pickling process), in which case an irregular attack would be obtained.

It appears self-evident that the invention can be extended to many alternative applications, all of which entail either one or a number of anodic pickling processes in solutions of lithium or magnesium salt. It is possible to modify the composition of the baths, to vary the concentration of the constituents or to add a number of different products such as surface-active agents, for example it is possible to carry out degreasing and/or anodic pickling operations (or anodic and cathodic pickling in alternate sequence), to vary the times, the current densities, the temperatures and the pH values in order to take into account both the composition and volume of parts being treated.

We claim:

1. A method of surface preparation of parts whose superficial portion at least consists of uranium or uranium-rich alloy, which method comprises making the part anodic in an aqueous solution of a salt selected from the group consisting of sulphates, acetates, citrates, tartrates, oxalatesn malates, lactates, formiates, melonates, pyrogallates and succinates of lithium and magnesium at current densities of from 2 to 20 A/dm.

2. A method according to claim 1, including preliminary steps of chemical degreasing, electrolytic degreasing and chemical etching.

3. A method of surface preparation of parts of uranium and uranium base alloys, comprising preliminary steps of chemical degreasing and electrolytic degreasing and further comprising an anodic electrolytic treatment in an aqueous solution of a salt selected from the group consisting of the organic salts and sulphate of lithium and magnesium, at current densities of from 2 to 20 A/dm'.

4. A method according to claim 3, wherein the preliminary steps further comprise etching with an aqueous solution of nitric acid.

5. A method in accordance with claim 3, wherein a salt selected from the group consisting of the acetates, citrates, tartrates, oxalates, malates, lactates, formiates, malonates, pyro gallates and succinates of lithium and magnesium is employed.

6. A method in accordance with claim 5, wherein the salt employed is lithium or magnesium acetate at a concentration of from 2 to 3 N.

7. A method in accordance with claim 5 wherein the temperature of the solution is between 10 C. and 50 C. and the duration of the anodic treatment is from 10 to 35 minutes.

8. A method in accordance with claim 6, wherein the current density is between 3 and 7 A/dm, the temperature is of the order of 25 C. and the duration of the treatment is comprised between 15 and 35 minutes. 

2. A method according to claim 1, including preliminary steps of chemical degreasing, electrolytic degreasing and chemical etching.
 3. A method of surface preparation of parts of uranium and uranium base alloys, comprising preliminary steps of chemical degreasing and electrolytic degreasing and further comprising an anodic electrolytic treatment in an aqueous solution of a salt selected from the group consisting of the organic salts and sulphate of lithium and magnesium, at current densities of from 2 to 20 A/dm2.
 4. A method according to claim 3, wherein the preliminary steps further comprise etching with an aqueous solution of nitric acid.
 5. A method in accordance with claim 3, wherein a salt selected from the group consisting of the acetates, citrates, tartrates, oxalates, malates, lactates, formiates, malonates, pyro gallates and succinates of lithium and magnesium is employed.
 6. A method in accordance with claim 5, wherein the salt employed is lithium or magnesium acetate at a concentration of from 2 to 3 N.
 7. A method in accordance with claim 5 wherein the temperature of the solution is between 10* C. and 50* C. and the duration of the anodic treatment is from 10 to 35 minutes.
 8. A method in accordance with claim 6, wherein the current density is between 3 and 7 A/dm2, the temperature is of the order of 25* C. and the duration of the treaTment is comprised between 15 and 35 minutes. 